This application claims the priority of German Patent Application No. 197 28 579.1, filed Jul. 4, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a vehicle air conditioning unit having a device for determining the dew point temperature of the outside air, an air duct, leading to a window of the vehicle, with a controllable closing device, a device for determining the temperature of the air blown through the ducts leading to the window of the vehicle, and a control device for automatically opening and closing the air duct leading to the window of the vehicle by appropriately energizing the air duct closing device.
Such air conditioning units are known, it being possible, for example, for the air duct leading to a window of a vehicle, to be an air duct which leads to a windshield and ends at the windshield in a so-called defroster nozzle. The primary function of such a defroster air duct usually is to remove windshield icing as rapidly as possible in the winter and to prevent renewed icing during a continuous driving operation. For this purpose, heated air is directed particularly over the defroster nozzles to the inside of the windshield. Furthermore, the defroster air duct of conventional air conditioning units frequently is intended to remove condensation from the inside of the windshield or to prevent its formation. For this purpose, the air conditioning unit then has a so-called reheat mode of operation, in which the air, taken in by the air conditioning unit, is cooled at the evaporator and thereby dried, and then subsequently the dried air is heated and blown as a dry and sufficiently warm air stream through the defroster air duct against the inside of the windshield.
Furthermore, the interior of the vehicle is air conditioned by such air conditioning units as a function of the so-called dew point temperature of the outside air and, possibly, also of the inside air. The dew point temperature is defined as the temperature below which condensation occurs. For example, if the temperature of the windshield falls below the dew point temperature of the outside air or of the inside air, condensation is deposited on the outside or inside of the windshield. Means for determining the dew point temperature are known in various versions. For example, in an air conditioning unit of this type, which is described in the German Patent document DE 36 24 171 A1, the combination of a temperature sensor and a moisture sensor for determining the dew point temperature is known. The dew point temperature of the outside air as well as that of the inside air is determined in the above-reference. This information is used, on the one hand, for selectively influencing the inside climate and, on the other, for identifying a threatening or already existing formation of condensation on the inside of the windshield. One of the functions of an associated air conditioning control device is to decide, on the basis of the information supplied by the sensor, whether the interior of the vehicle is to be ventilated by way of an air duct directed at the windshield or by way of an air duct blowing more air into the central region of the interior.
The outside air taken in, which is cooled, for example, using an evaporator, is generally blown only over a portion of the existing air ducts into the interior by conventional air-conditioning units during the cooling operation, that is, during the operating phase for cooling the interior air. In particular, this is done by air conditioning units with automatic air guidance, that is, automatic division of the incoming air among the different air ducts present. During this time in particular, the defroster air duct generally remains closed in order to prevent the dangerous occurrence of condensation on the outside of the windshield by excessive cooling of the latter. In the case of air conditioning units with the possibility of automatically opening and closing the defroster air ducts, the changeover points are selected so that the windshield remains protected against condensation on the outside even during extremely moist and hot weather, during which there is the greatest tendency for condensation to form on the outside of the windshield. On the other hand, for the cooling operation, it is desirable that the incoming air is blown from as many air ducts as possible into the interior of the vehicle. After all, if fewer air ducts are open, the velocities of the blown air are greater and, as a result, there is a greater draft.
The invention is therefore based on the technical problem of making available a vehicle air-conditioning unit of the type described above, with which, on the one hand, condensation on the outside of the windshield during cooling operations is reliably prevented and, on the other hand, as far as possible draft-free ventilation of the interior is achieved.
Pursuant to the invention, this problem is solved by making available a vehicle air-conditioning unit wherein in the cooling operation of the air conditioning unit, the control device automatically opens and closes the air duct leading to the window of the vehicle as a function of the dew point temperature determined for the outside air and of the temperature determined for the air blown out through this air duct, the air duct being kept closed by the control device as long as the temperature of the air blown out is lower than the dew point temperature of the outside air plus a specifiable safety offset temperature, and kept open at least partially as long as the temperature of the air blown out lies above the dew point temperature of the outside air plus the specifiable offset temperature value.
With this unit, the air duct, leading to a glass pane of the vehicle, particularly to the windshield, is opened and closed during the cooling operation of the unit by the control device automatically as a function of the determined dew point temperature of the outside air and of the determined temperature of the air that is blown in this air duct in the direction of the window pane of the vehicle. Moreover, the control device keeps this air duct closed as long as the temperature of the blown air is lower than the sum of the dew point temperature of the outside air and a specifiable safety offset temperature. Depending on the particular case, the latter can be set at a desirable, suitable value greater than zero, in order to ensure that the air duct is closed in good time before condensation appears on the outside of the glass pane, when the temperature of the blown air approaches the dew point temperature of the outside air from above. Otherwise, the control device keeps the air duct in question at least partially open at all times, as long as the temperature of the air that is blown is higher than the sum of the dew point temperature of the outside air and the specifiable safety offset temperate.
All told, a cooling operation for which even the air duct leading to the window pane of the vehicle contributes in every situation towards achieving a ventilation of the interior with cooling air which is as draft-free as possible, and therefore a ventilation over as many as possible of the existing air ducts so long as this does not lead to condensation on the outside of the window pane, can consequently be realized with this air-conditioning unit. In particular, this air duct can be a defroster air duct. Compared to conventional air-conditioning units with a fixed changeover point between opening and closing the defroster air duct during the cooling operation, which changeover point depends on the most unfavorable case of extreme moist and hot weather, the air duct leading to the window pane of the vehicle can, in the case of the inventive air-conditioning unit, remain open longer during the cooling operation, for example, when the weather is less moist. This increases the comfort of the interior cooling for the passengers of the vehicle.
In the case of a preferred embodiment of the air-conditioning unit the air duct, leading to a window pane of the vehicle, can be adjusted within a temperature interval, the width of which can be specified and which adjoins the dew point temperate of the outside air plus the specifiable safety offset temperature, infinitely variably or in steps to intermediate positions between its completely open and its completely closed positions. With that, the air duct can still be used at least partly in this range for ventilating the interior with cool air, while at the same time its air throughput is limited so that the occurrence of condensation on the outside of the window panes is reliably prevented.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.